Bebe King, 6, Elsie Dot Stancombe, 7, and 9-year-old Alice da Silva Aguiar died in the stabbing spree, carried out when Rudakubana was still 17.
Speaking to the inquiry from an undisclosed remote location, Alphonse Rudakubana said his son had violent “outbursts” which could happen twice a day.
“It was much much more than when he was in fulltime education.”
In one incident, Rudakubana said his son poured oil over his head.
Rudakubana said Axel then poked his chest and said: “‘If you get me out of here, this house, it may take a day, it may take a week, maybe a month, maybe years, I will kill you and trust me I will kill you’.
“He looked serious at me and actually it was quite frightening for me.”
Rudakubana said he or his wife reported some incidents to the authorities, such as his son’s Child and Adolescent Mental Health Services team, but not all.
When asked why, he said he was worried his son would be taken away from him.
Earlier, Axel’s brother Dion said his father told him to be “careful” around his younger brother days before the attack.
The inquiry, which can compel witnesses to give evidence, is chaired by a retired senior judge. Its recommendations are not legally binding, but may impact future policy.
-Agence France-Presse